1. Field of the Invention
The present invention relates to improvement in a hollow stabilizer which is used for a vehicle such as a motor vehicle or the like.
2. Description of the Related Art
Conventionally, a spring member has been used as a stabilizer for increasing a roll rigidity of a vehicle. The stabilizer is provided with a torsion portion which extends in a width direction of the vehicle, arm portions which are positioned in both side ends of the torsion portion, and a curved portion which connects both of the torsion portion and the arm portions to a shoulder portion between the torsion portion and the arm portions. Furthermore, the curved portion may be provided not only in the shoulder portion but also in the arm portions and the torsion portion.
In the stabilizer mentioned above, loads in vertically opposite directions to each other are input to both of the arm portions at a time when the vehicle travels on a curved road, whereby both of the arm portions are bent in directions opposite to each other. Thus, the torsion portion is twisted, and a rolling of the vehicle body is inhibited by a repulsive force which is generated at this time. Therefore, a bending stress is mainly generated in the arm portions, and a torsional stress is mainly generated in the torsion portion. Furthermore, a combined stress between the bending stress and the torsional stress is generated in the shoulder portion. Furthermore, a solid round bar is used as a material of the conventional stabilizer; however, there a hollow stabilizer is known employing a pipe member for the purpose of weight reduction of the vehicle.
In this case, a fuel tank, an exhaust pipe, a frame and various kinds of rods are placed under a floor of the vehicle, and in order to pass the stabilizer through gaps between such parts, the stabilizer may often be bent in a three-dimensional manner by forming a plurality of curved portions in the torsion portion, the arm portions and/or the shoulder portion.
A bending method for the conventional hollow stabilizer is shown in FIG. 5. A bending apparatus 50 is provided with a bending mold 52 having a circular arc forming surface 51, a clamp mold 53, and a pressure mold 54 which press a stabilizer main body 11. The bending mold 52 is rotated in a direction of an arrow in the drawing by a rotation driving mechanism (not shown), and is constructed so as to bend the stabilizer main body 11 in a circular arc shape having a center bending radius R along the forming surface 51.
The stabilizer is bent in a three-dimensional manner for the purpose of passing through the gaps between the various kinds of parts in the vehicle. However, in order to bend in a three-dimensional manner by using the conventional bending mold, a plurality of two-dimensional bending processes are combined. In the case of the conventional bending mold, due to the shape of the bending mold, the bending process can be performed only by providing a straight portion between one curved portion and another curved portion. As a result, this length of material is useless. Furthermore, in order to have the stabilizer accommodated in the gap in which the other parts are provided in a complicated manner, it is necessary to provide a plurality of curved portions in the stabilizer so as to avoid interference with the parts. Accordingly, the number of bending processes is increased, and the number of the straight portions between the curved portions is also increased. However, it is often the case that it is difficult to receive the long straight portion in the gap in which the parts are complicated, and the necessity of bending the stabilizer little by little arises. As a result, it is impossible to set a large bending radius, and when the bending radius of the hollow stabilizer is made small, there is a problem that the pipe is flattened and of uneven thickness. If the pipe is flattened and of unevenly thickness, a stress concentration is generated, and therefore durability is reduced. Furthermore, in the case of performing the bending process for different diameters using the conventional bending mold, it is necessary to prepare numerous bending molds having different bending diameters, and a large amount of labor is required for controlling the bending molds.